Geothermal Energy.pdf

BMM 4753 RENEWABLE ENERGY Topic 3 GEOTHERMAL ENERGY Summary 1. Introduction 2. Geothermal energy - Types of resources 3. Environmental impact of geothermal power plants 4. Operational difficulties of geothermal plants 5. Estimation of GE availability (Hot dry rock) 6. Geothermal Energy in Malaysia 7. New Developments

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.1 Introduction Earth

The Earth has a number of different layers a. The core itself has two layers o inner core made of solid iron o outer core made of very hot melted rock, called magma.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.1 Introduction Earth

b. The mantle surrounds the core o and is about 1,800 miles thick. o made up of magma and rock.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.1 Introduction Earth

c. The crust is the outermost layer of the Earth o the land that forms the continents and ocean floors o 3 to 5 miles thick under the oceans o 5 to 35 miles thick on the continents.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.1 Introduction Tectonic Plate Formation

Earth's crust is broken into pieces called plates o Magma comes close to the Earth's surface near the edges of these plates where volcanoes occur. o The lava that erupts from volcanoes is partly magma. Deep underground, the rocks and water absorb the heat from this magma.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.1 Introduction Earth Temperature

o The temperature of the rocks and water gets hotter o hotter as it gets deeper underground.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.1 Introduction Geothermal Resources Geothermal are renewable energy sources which utilise the heat within the earth to create either a source of renewable heat or renewable electricity. i. Geothermal energy is generated in the Earth's core, mostly from the decay of naturally radioactive materials like uranium and potassium. The amount of heat within 10,000 meters of the surface contains 50,000 times more energy than all the oil and natural gas resources in the world. ii. The areas with highest underground temperatures are in regions with active or geologically young volcanoes. These "hot spots" occur at plate boundaries or at places where the crust is thin enough to let the heat through. The Pacific Rim, called the "ring of fire" for all of its volcanoes, has many hot spots, including some in Alaska, California, and Oregon.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.1 Introduction Geothermal Resources iii. These regions are also seismically active. The many earthquakes and the movement of magma break up the rock covering, allowing water to circulate. As the water rises to the surface, natural hot springs and geysers occur, such as "Old Faithful" at Yellowstone National Park. The water in these systems can be more than 200oC. iv. The current production of geothermal energy from all uses place third among renewables, following hydroelectricity and biomass, and ahead of solar and wind. Despite these impressive statistics, the current level of geothermal use pales in comparison to its potential. The key to wider geothermal use is greater public awareness and technical support--two areas in which the Geo-Heat Center is very active.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.2 Types of Resources

Current Commercial Utilization i. Hydrothermal Reservoirs ii. Earth Energy Advanced Technologies Yet To Be Developed iii. Hot Dry Rock iv. Geopressured Brines v. Magma

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.2 Types of Resources i.

Hydrothermal Reservoirs

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.2 Types of Resources i.

Hydrothermal Reservoirs

❖ Hydrothermal reservoirs are large pools of steam or hot water trapped in porous rock. To generate electricity, the steam or hot water is pumped to the Earth's surface where it drives a turbine that spins an electric generator. Because steam resources are rare, hot water is used in most geothermal power plants. Steam and hot water power plants use different power production technologies.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.2 Types of Resources i.

Hydrothermal Reservoirs

❖ These resources can be classified as low temperature (less than 90°C), moderate temperature (90°C- 150°C), and high temperature (greater than 150°C). The uses to which these resources are applied are also influenced by temperature. The highest temperature resources are generally used only for electric power generation. Current U.S. geothermal electric power generation totals approximately 2200 MW or about the same as four large nuclear power plants. Uses for low and moderate temperature resources can be divided into two categories: direct use and ground-source heat pumps.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.2 Types of Resources i.

Hydrothermal Reservoirs

a) Dry Steam Steam is routed directly to the turbines, eliminating the need for the boilers used by conventional natural gas and coal plants. b) High-temperature Hot Water Hot water with temperatures above 200°C are usually utilized using a flash technology where hot water is sprayed into a lowpressure tank. The water vaporizes to steam, which is routed to the turbine. c) Moderate-temperature Hot Water—hot water resources below 200°C are utilized using a binary cycle technology where the hot water vaporizes a secondary working fluid, which then drives the turbine.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.2 Types of Resources ii. Earth Energy ❖ Direct use, involves using the heat in the water directly (without a heat pump or power plant) for such things as heating of buildings, industrial processes, greenhouses, aquaculture (growing of fish) and resorts. Direct use projects generally use resource temperatures between 38°C to 149°C . Current U.S. installed capacity of direct use systems totals 470 MW or enough to heat 40,000 average-sized houses. ❖ The heat contained in shallow ground—is used to directly heat or cool homes and commercial buildings through "direct-use" technologies such as geothermal heat pumps (GHP) and district heating systems. Unlike other forms of geothermal energy, earth energy is found throughout the U.S.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.2 Types of Resources ii. Earth Energy a) Geothermal Heat Pumps (GHP) * Ground-source heat pumps use the earth or groundwater as a heat source in winter and a heat sink in summer. Using resource temperatures of 4°C to 38°C, the heat pump, a device, which moves heat from one place to another, transfers heat from the soil to the house in winter and from the house to the soil in summer. Accurate data is not available on the current number of these systems; however, the rate of installation is thought to be between 10,000 and 40,000 per year.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.2 Types of Resources ii. Earth Energy a) Geothermal Heat Pumps… cont’d * GHPs use the Earth's relatively constant ground temperature to provide low-cost heating and cooling. In the winter, GHPs transfer heat from the ground into homes and buildings; in the summer, GHPs cool homes and buildings by transferring indoor heat into the ground. GHPs can cut heating costs by 50 percent, and cooling costs by 25 percent. More than 200,000 GHPs are operating in U.S. homes, schools and commercial buildings.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.2 Types of Resources ii. Earth Energy b) District Heating Systems Many communities use district-heating systems to heat homes or public buildings by circulating hot water through pipes. Overall, direct-use applications use geothermal energy to supply the energy equivalent of nearly 1 million barrels of oil.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.2 Types of Resources iii. Hot Dry Rock (HDR)

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.2 Types of Resources iii. Hot Dry Rock a) This energy consists of dry, impermeable rock. To use this energy, water must be pumped into the rock at high pressures to widen existing fissures and create an underground reservoir of steam or hot water b) Geothermal heat occurs everywhere under the surface of the earth, but the conditions that make water circulate to the surface are found only in less than 10% of the land area of the earth. An approach to capturing the heat in dry areas is known as "hot dry rock." The rocks are first broken up by pumping high pressure water through them. Water is then pumped from the surface down through the broken hot rocks. After the water heats up, it is brought back to the surface through a second well and used to drive turbines or to provide heat.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.2 Types of Resources iii. Hot Dry Rock c) Researchers at the Los Alamos National Lab in New Mexico have studied hot dry rock since 1974. The Fenton Hill plant involves a well drilled 3,500 m into rock at 220oC. Water pumped down the well at 30oC returned to the surface at 180oC. The plant has produced as much as 5 megawatts of power, proving the technical feasibility of hot dry rock. d) However, a number of barriers must be overcome before hot dry rock can become a commercial source of power. The wells must be quite deep, deeper than for conventional geothermal plants. Also, the flow of heat through dry rock is slow, which means the heat removed through the well will be slow to be renewed. Finally, the most promising sites for hot dry rock are in dry areas of the West, which means that water may be hard to come by.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.2 Types of Resources iv. Geopressured Brines These brines are hot, pressurized waters containing dissolved methane. Both the heat and methane can be used for power generation.

v. Magma Magma is the molten or partially molten rock found below the Earth's crust. Magma reaches temperatures up to 1200°C. While some magma bodies exist at accessible depths, a practical way to extract magma energy has yet to be developed.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.2 Types of Resources Tectonic Plate Location

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.2 Types of Resources World Geothermal Power Plants

People living in these areas are receiving electricity from geothermal power plants.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.2 Types of Resources World Geothermal Power Production (8,217MW) United States Philippines Italy Mexio Indonesia Japan New Zealand Iceland Costa Rica El Salvador Nicaragua Kenya China Turkey Portugal Russia Guatemala France Taiwan Thailand Zambia

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BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.3 Environmental Impact of Geothermal Power Plants Applications i.

Worldwide, direct uses of geothermal water avoids the combustion of fossil fuels equivalent to burning of 830 million gallons of oil or 4.4 million tons of coal per year. ii. Worldwide electrical production from geothermal reservoirs avoids the combustion of 5.4 billion gallons of oil or 28.3 million tons of coal iii. Geothermal direct use facilities have minimal impacts on the environment. iv. Geothermal power plants are relatively easy on the environment. They are successfully operated in the middle of crops, in sensitive desert environments and in forested recreation areas.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.3 Environmental Impact of Geothermal Power Plants Applications Direct Use Geothermal Water 1. Hot Spring / Bathing 2. Agriculture 3. Aquaculture 4. Industry 5. Heating/District Heating Indirect Use Geothermal Steam/water 6. Electrical Power Generation

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.3 Environmental Impact of Geothermal Power Plants 1. Hot Spring Bathing

Since Roman times, we have piped the hot water into pools to better control the temperature. These are photos of outdoor and indoor pool and spa bathing in Japan, the US, and Europe

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.3 Environmental Impact of Geothermal Power Plants 2. Agriculture

Peppers, tomatoes, and flowers are commonly grown in geothermally heated greenhouses

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.3 Environmental Impact of Geothermal Power Plants 3. Aquaculture

Geothermal water is also used to speed the growth of fish. These are growing in a geothermally heated hatchery at Mammoth Lakes, California

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.3 Environmental Impact of Geothermal Power Plants 4. Industry

This plant operates in the middle of crops in the Imperial Valley, California. High mineral contents of some southern California geothermal reservoirs provide salable byproducts like silica and zinc

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.3 Environmental Impact of Geothermal Power Plants 5. District Heating

Hot water from one or more geothermal wells is piped through a heat exchanger plant to heat city water in separate pipes. Hot city water is piped to heat exchangers in buildings to warm the air

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.3 Environmental Impact of Geothermal Power Plants 6. Electrical Power Generation

The first modern geothermal power plants built in Lardello, Italy. They were destroyed in World War II and rebuilt. Today after 90 years, the Lardello field is still producing.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.3 Environmental Impact of Geothermal Power Plants 6. Electrical Power Generation

The first geothermal power plants in the U.S. were built in 1962 at The Geysers dry steam field, in northern California. It is still the largest producing geothermal field in the world

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.3 Environmental Impact of Geothermal Power Plants 6. Electrical Power Generation

This power plant provides about 25% of the electricity used on the Big Island of Hawaii.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.4 Operational Difficulties Of Geothermal Power Plants Direct Use of Geothermal Heat ❑ Heat from geothermal springs can also be used directly for heat. Hot spring water is used to heat greenhouses for plants, to dry out fish and deice roads, for improving oil recovery, and to heat fish farms and spas. In Klamath Falls, Oregon, and Boise, Idaho, geothermal water has been used to heat homes and buildings for over a century. New housing developments in Reno, Nevada, are using geothermal heat from a well to heat homes. ❑ In Iceland, virtually every building in the country is heated with hot spring water. In fact, Iceland gets 45% of its energy from geothermal sources. In Reykjavik, for example (population 145,000), hot water is piped in from 25 kilometers away, and residents use it for heating and for hot tap water.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.4 Operational Difficulties Of Geothermal Power Plants How Geothermal Energy Is Captured i.

The most common current way of capturing the energy from geothermal sources is to tap into naturally occurring "hydrothermal convection" systems. When heated water is forced to the surface, it is a relatively simple matter to capture that steam and use it to drive electric generators. Geothermal power plants drill their own holes into the rock to more effectively capture the steam.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.4 Operational Difficulties Of Geothermal Power Plants How Geothermal Energy Is Captured ii. There are three designs for geothermal power plants, all of which pull hot water and steam from the ground, use it, and then return it as warm water to prolong the life of the heat source. In the simplest design, the steam goes directly through the turbine, then into a condenser where the low-temperature steam is condensed into water. In a second approach, the steam and hot water are separated as they come out of the well; the steam is used to drive the turbine while the water is sent directly back underground.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.4 Operational Difficulties Of Geothermal Power Plants How Geothermal Energy Is Captured iii. In the third approach, called a binary system, the hot water and steam mixture is passed through a heat exchanger, where it heats a second liquid (like isobutane) in a closed loop. The isobutane boils at lower temperatures than water, so as steam it is used to drive the turbine. The three systems are shown in the graphics here.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.4 Operational Difficulties Of Geothermal Power Plants How Geothermal Energy Is Captured iv. The choice of which design to use is determined by the resource. If the water comes out of the well as steam, it can be used directly, as in the first design. If it is hot water, it must go through a heat exchanger. Since there are more hot water resources than pure steam, there is more growth potential in the heat exchanger design.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.4 Operational Difficulties Of Geothermal Power Plants How Geothermal Energy Is Captured v. The largest geothermal system now in operation is a steamdriven plant in an area called The Geysers, north of San Francisco. Despite the name, they’re actually no geysers here, and the heat that is used for energy is all steam, not hot water. Although the area was known for its hot springs as far back as the mid-1800s, the first well for power production was drilled in 1924. Deeper wells were drilled in the 1950s, but real development didn't occur until the '70s and '80s. By 1990, 26 power plants had been built, for a capacity of over 2,000 megawatts. In 1992, the area produced enough power for a city of 1.3 million.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.4 Operational Difficulties Of Geothermal Power Plants How Geothermal Energy Is Captured vi. Because of the rapid development of the area in the '80s, and the technology used, the steam resource has been declining since 1988. In the Geysers, the plants use an evaporative watercooling process to create a vacuum that pulls the steam through the turbine, producing power more efficiently. But this process loses 60 to 80 percent of the steam to the air, not reinjecting it underground. While the steam pressure may be declining, the rocks underground are still hot. Some efforts are under way to remedy the situation, including reinjecting water pumped in through a 26-mile pipeline, and replacing the water-cooled systems with air-cooled.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.4 Operational Difficulties Of Geothermal Power Plants How Geothermal Energy Is Captured vii. Another problem with open systems like the ones at the Geysers is that they produce some air emissions. Hydrogen sulfide, along with small amounts of arsenic and minerals, is released in the steam. At a power plant at the Salton Sea reservoir in California, a significant amount of salt builds up in the pipes and must be removed. While the plant initially started to put the salts into a landfill, they now reinject the salt back into a different well. With closed-loop systems, such as the binary system, there are no emissions; everything brought to the surface is returned underground.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.4 Operational Difficulties Of Geothermal Power Plants How Geothermal Energy Is Captured - Cooper Basin, Australia

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.5 Estimation of Geothermal Energy Availability Hot Dry Rock Dry Rock And Hot Aquifer Analysis Consider a large mass of dry material extending from near the earth’s surface to deep inside the crust

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.5 Estimation of Geothermal Energy Availability Hot Dry Rock

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.5 Estimation of Geothermal Energy Availability Example Problem Hot Dry Rock

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.5 Estimation of Geothermal Energy Availability Example Problem Hot Dry Rock

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.5 Estimation of Geothermal Energy Availability Example Problem Hot Dry Rock

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.5 Estimation of Geothermal Energy Availability Example Problem Hot Dry Rock

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.5 Estimation of Geothermal Energy Availability Example Problem Hot Dry Rock

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.5 Estimation of Geothermal Energy Availability Example Problem Hot Dry Rock

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.6 Geothermal Energy in Malaysia Tawau in Malaysia has an electricity generation potential of up to 67 MW from geothermal resources following the discovery of a geothermal site in Apas

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Energy 3.6 Geothermal Energy in Malaysia a) In Malaysia, Tenaga Nasional Bhd (TNB) has identified four potential geothermal power generation sites that could collectively generate more than 2 MW of electricity, at a confidence level of 60%. The locations of the four sites in the peninsula area were not disclosed. Once testing raises the confidence level to 90%, TNB would begin drilling. Expectations are that the projects to be on line by 2016. TNB’s move into geothermal energy was part of its long-term strategy to diversify into renewable energy sources, given the environment of rising coal and gas prices. TNB had earmarked four sites to set up geothermal power plants which will utilise steam generated from hot springs. There are more than 40 thermal springs in Peninsular Malaysia. Most of these springs are good potential sites to generate geothermal power as part of the nation’s plan to enhance its renewable energy potential.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.6 Geothermal Energy in Malaysia b) Each geothermal plant would have the potential to generate more than two megawatts of electricity. A large hot spring can generate up to 20 megawatts of power. The need to look at other viable renewable energy options is gaining -importance, given adverse public opinion on the use of nuclear power, post-nuclear disaster in Japan. The foray into geothermal power is in line with the government’s plan to increase renewable energy’s contribution to Malaysia’s power generation mix from less than one per cent currently to 5.5 per cent (985MW) by 2015. Tawau has an electricity generation potential of up to 67 MW from geothermal resources following the discovery of a geothermal site in Apas by a study by the Mineral and Geoscience Department. A study has found a reservoir about 2,000 to 3,000m below the earth’s surface with water at temperatures of 220 to 236 degrees Celsius which was more than enough heat to generate electricity.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.7 Recent Developments 1. Comparing statistical data for end-1996 (SER 1998) and the present Survey, there has been an increase in world geothermal power plant capacity (+9%) and utilisation (+23%) while direct heat systems show a 56% additional capacity, coupled with a somewhat lower rate of increase in their use (+32%). 2. Geothermal power generation growth is continuing, but at a lower pace than in the previous decade, while direct heat uses show a strong increase compared to the past. 3. Six countries with the largest electric power capacity are: USA with 2 228 MWe is first, followed by Philippines (1 863 MWe); four countries (Mexico, Italy, Indonesia, Japan) had capacity (at end-1999) in the range of 550-750 MWe each. These six countries represent 86% of the world capacity and about the same percentage of the world output, amounting to around 45 000 GWhe.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.7 New Developments 4. Strong decline in the USA in recent years, due to overexploitation of the giant Geysers steam field, has been partly compensated by important additions to capacity in several countries: Indonesia, Philippines, Italy, New Zealand, Iceland, Mexico, Costa Rica, El Salvador. Newcomers in the electric power sector are Ethiopia (1998), Guatemala (1998) and Austria (2001). Total of 22 nations are generating geothermal electricity sufficient for 15 million houses. 5. Three countries with the largest amount of installed power: USA (5 366 MWt), China (2 814 MWt) and Iceland (1 469 MWt) cover 58% of the world capacity, which has reached 16 649 MWt, enough to provide heat for over 3 million houses. Out of about 60 countries with direct heat plants, beside the three above-mentioned nations, Turkey, several European countries, Canada, Japan and New Zealand have sizeable capacity.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.7 New Developments 6. With regard to direct use applications, a large increase in the number of GHP installations for space heating (presently estimated to exceed 500 000) has put this category in first place in terms of global capacity and third in terms of output. Other geothermal space heating systems are second in capacity but first in output. Third in capacity (but second in output) are spa uses followed by greenhouse heating. Other applications include fish farm heating and industrial process heat. The outstanding rise in world direct use capacity since 1996 is due to the more than two-fold increase in North America and a 45% addition in Asia. Europe also has substantial direct uses but has remained fairly stable: reductions in some countries being compensated by progress in others.

BMM 4753 RENEWABLE ENERGY Topic 3 Geothermal Thermal Energy 3.7 New Developments 7. In R&D, the hot dry rock (HDR) project at Soultz-sous-Forêts near the French-German border has progressed significantly. Besides the ongoing Hijiori site in Japan, another HDR test has just started in Switzerland (Otterbach near Basel). 8. The total world use of geothermal power is giving a contribution both to energy saving (around 26 million tons of oil per year) and to CO2 emission reduction (80 million tons/year if compared with equivalent oil-fuelled production).